In the high-speed optical fiber communication, light waveform may sometimes deteriorate limiting transmission speed and transmission distance due to a characteristic of wavelength dispersion in the fiber. Wavelength dispersion (hereinafter referred to as “dispersion”) means the wavelength dependence of group velocity at which signals propagate in the optical fiber. Strictly speaking, light waveform has a plurality of wavelength components (the spectrum spreads out), and when the group velocity is dependent on wavelength, there appear some components that advance slowly in the fiber and some other components that advance quickly, and as a result the waveform expands, and if the dispersion value is not negligible, the waveform is deformed and receiving characteristics deteriorates. As dispersion amount is proportionate to the fiber length, transmission distance is limited as a consequence.
Generally, dispersion amount depends on the type and distance of the optical fiber. To cite an example of numerical value, in the case of the most ordinary SMF (single mode fiber) as laid optical fiber, for example, the dispersion amount per km is about 17 ps/nm/km.
In a transmission system of 10 Gbps, the dispersion tolerance of optical signals is approximately 1,000 ps/nm, and when the transmission line length exceeds 60 km, it is sometimes impossible to receive under the impact of waveform deformation. As the effect of dispersion is in inverse proportion to the square of the signal bit rate, for example in a transmission system of 40 Gbps, the dispersion tolerance of optical signals is reduced to one sixteenth ( 1/16) of the value mentioned above, and it is often difficult to realize the transmission of more than several kilometers without countermeasures against dispersion.
A traditional method that has been used generally to avoid the effect of waveform deformation due to dispersion is the application of a dispersion compensator. The dispersion compensator is an optical device whose dispersion amount is inverse to the dispersion amount of the optical fiber used in the transmission line. This dispersion compensator cancels dispersion in the optical fiber enabling to contain the waveform deformation resulting from dispersion.
As this dispersion compensator, dispersion compensation fiber (DCF) is one of the devices most frequently used. The DCF is a device designed to maintain a dispersion property inverse to that of the optical fiber in the transmission line by devising on the material and construction of the fiber. Among various types of dispersion compensator, one type cancels dispersion at a specific wavelength and another type cancels the dispersion wavelength dependency (dispersion slope value) of the optical fiber in the transmission line. In addition, generally, the dispersion compensation amount of the DCF is determined by the length of the DCF. Therefore, in this case, when the fiber length is fixed upon determining its length once, this dispersion compensation amount is fixed. Such a dispersion compensator is called “a fixed dispersion compensator” in the sense that the dispersion amount is fixed.
As this fixed dispersion compensator, in addition to the above-mentioned DCF, fiber grating is also generally used. The fiber grating is a device made by irradiating an optical fiber with ultraviolet rays so that a refraction index changing structure is formed by the light wavelength order within the optical fiber. The refraction index changing structure behaves like a diffraction grating and acts as a reflection mirror at a specified wavelength. It is possible to adjust the amount of delay at the time of reflection for each wavelength by forming this refraction index changing structure in such a way that the cycle may be narrow down (or widen) in the axial direction of the optical fiber. Therefore, it will be possible to cancel the dispersion characteristic of the optical fiber in the transmission line by adequately designing this cycle. Such a fiber grating susceptible of dispersion compensation is called “chirped fiber grating (CFBG).
However, in a super high-speed transmission system of 40 Gbps, in view of a very narrow range of dispersion tolerance of 100 ps/nm or less for example as mentioned above, it is normally difficult to finely adjust dispersion compensation by means of a fixed dispersion compensator adapting to the length of the transmission fiber. In addition, when the WDM transmission system is taken into consideration, it is necessary to take into account not only the dispersion amount but also the wavelength dependency of the dispersion amount itself called “dispersion slope value” (difference in dispersion amount for each signal wavelength among the WDM signals). In case of an attempt to adjust the dispersion amount to be compensated for each wavelength in this way by means of a fixed dispersion compensator, it is necessary to prepare in advance a large number of fixed dispersion compensators of different compensation amount, and this constitute an problem regarding cost. Although there is a DCF designed to compensate even dispersion slope value as mentioned above, it is normally difficult to compensate completely dispersion slope value. For example, in a transmitter having a stringent dispersion tolerance of 40 Gbps, a dispersion adjustment for each wavelength is still required and it is difficult to realize dispersion compensation with a fixed dispersion compensator.
On the other hand, as a variable dispersion compensator with variable compensation amount, the above-mentioned method of forming a temperature grading in the axial direction of CFBG is known. Dispersion amount becomes variable when this temperature grading is controlled.
FIG. 18 shows the configuration for the control of the traditional variable dispersion compensator.
In order to control the dispersion amount of a variable dispersion compensator to the optimum value, an adaptive control circuit is necessary. An example of this control circuit is, for example, the configuration shown in Invention by Benjamin John Eggleton: “Optical communication system provided with an automatic dispersion compensation module”, JP-A No. 244394/2000. This adaptive control circuit includes a variable dispersion compensator (101), a variable dispersion compensator (102), an optical filter (103), an OE (104), the recovery unit of a data clock (105), error detection unit (106), error rate computing unit (107), control circuit (108) and control circuit (109).
In the same configuration, optical data are converted into electric signals by the OE (104) after passing through an invariable dispersion compensator (101, and a variable dispersion compensator (102). The error detecting unit (106) detects signal errors and the error rate computing unit (107) computes the error rate of electric signals being received. From the change in this error rate, the new control value is computed by the control circuit (108) and the control circuit (109) to control said variable dispersion compensator (101) or the variable dispersion compensator (102).
And other variable dispersion compensators are described in the non-patent documents 2, 3 and the like as described below.